The present disclosure generally relates to corrosion and, more specifically, to methods and systems for suppressing corrosion of titanium surfaces, particularly during subterranean treatment operations.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Corrosive environments comprising an acid can cause severe corrosion damage to many types of metal surfaces. As used herein, the term “corrosion” and grammatical variants thereof will refer to any reaction between a metal surface and its surrounding environment that causes a deterioration or change in the metal surface's properties or morphology. Examples of corrosion damage to a metal surface include, but are not limited to, rusting, metal dissolution or erosion, pitting, peeling, blistering, patina formation, cracking, embrittlement, and any combination thereof.
Acidic treatment fluids are frequently utilized in the course of conducting various subterranean treatment operations. Illustrative uses of acidic treatment fluids during subterranean treatment operations include, for example, matrix acidizing of siliceous and/or non-siliceous formations, scale dissolution and removal operations, gel breaking, acid fracturing, and the like. When acidizing a non-siliceous material, such as a carbonate material, mineral acids such as hydrochloric acid may often be sufficient to affect dissolution. Organic acids may be used in a similar manner to hydrochloric acid when dissolving a non-siliceous material. Siliceous materials, in contrast, are only readily dissolvable using hydrofluoric acid, optionally in combination with other acids, to provide a solubility-promoting low-pH state. Illustrative siliceous materials can include, for example, silica, silicates, aluminosilicates, and any combination thereof, optionally in further combination with a non-siliceous material, such as a carbonate material.
Corrosion of metal surfaces within a wellbore penetrating a subterranean formation, such as tubulars and tools, for example, can be highly undesirable due to the difficulty, cost, and production downtime associated with replacing or repairing such components. In many instances, elevated temperatures within subterranean formations can dramatically accelerate downhole corrosion rates. Regardless of its location, corrosion-induced damage of a metal surface can represent a significant safety and/or environmental concern due to potential well failure issues.
Metal surfaces in fluid communication with a wellbore can similarly be susceptible to corrosion and its undesirable effects. Outside the wellbore, corrosion can occur during introduction of a treatment fluid to the wellbore, during production, or any combination thereof. In subsea wellbores, for example, a subsea riser structure extending from the wellbore (e.g., via a blowout preventer) to a platform or vessel on the ocean's surface or just below the ocean's surface can be susceptible to corrosion during production of a partially spent acidic fluid from the wellbore. The risk of corrosion to various components of a subsea riser structure can be so significant that exclusion of potentially corrosive agents from the wellbore system may be warranted, possibly limiting the realm of treatment operations that are available to a well operator.
Organic corrosion inhibitors may be used to mitigate the corrosive effects of some mineral and organic acids, but not all, and numerous limitations exist. Certain metals are also more susceptible to the effects of corrosion than are others, and successful corrosion inhibitor strategies for one metal do not necessarily work for another. As used herein, the terms “inhibit,” “inhibitor,” “inhibition” and other grammatical forms thereof will generally refer to the lessening of the tendency of a phenomenon to occur and/or the degree to which that phenomenon occurs. The terms “suppress,” “suppression” and other grammatical forms thereof may be used equivalently herein. The term “inhibit” and equivalents thereof do not imply any particular extent or amount of inhibition or suppression unless otherwise specified herein.
Hydrofluoric acid- and acidic fluoride-containing fluids can be especially corrosive toward certain types of sensitive metal surfaces, such as those containing titanium or a titanium alloy. Titanium and titanium alloys are lightweight, strong and resistant to most formation fluids and a great number of common treatment fluids, including those containing organic acids and/or mineral acids such as hydrochloric acid. However, titanium and titanium alloys are especially prone to corrosion by even modest quantities of hydrofluoric acid or fluoride ions at pH values of about 7 or less. Moreover, conventional organic corrosion inhibitors are not especially effective for titanium and titanium alloys. Without being bound by any theory or mechanism, it is believed that the extreme reactivity of titanium toward these fluids is due to removal of a passivating surface oxide by hydrofluoric acid. The extreme sensitivity of titanium and titanium alloys to hydrofluoric acid can preclude the use of hydrofluoric acid where such metals are in fluid communication with a wellbore, thereby limiting one's ability to acidize a siliceous material. For example, titanium and titanium alloys are frequently used in expansion or stress joints of subsea riser structures, which can make stimulation operations very difficult to conduct in deepwater wellbores containing a siliceous material.
Although inhibited titanium alloys (e.g., Ti Grade 29 alloy, which is inhibited by small amounts of ruthenium, or Ti Grade 7 alloy, which is inhibited by small amounts of palladium) can display a decreased propensity toward corrosion in the presence of hydrofluoric acid than do pristine titanium or uninhibited alloys (e.g., commercially pure titanium, CP-Ti), corrosion is often still an issue. Moreover, cost and sourcing of inhibited titanium alloys can be problematic, especially for large-scale operations.